Switch mode power supplies are used in a wide variety of household and industrial appliances that require a regulated direct current (DC) voltage for their operation. There are a wide variety of known DC-DC converter topologies using PWM (pulse width modulation) or PFM (pulse frequency modulation) control to regulate output voltage.
One type of DC-DC converter topology is a resonant switched mode power converter. A resonant converter included in a resonant switched mode power converter controller with PFM utilizes resonant properties to provide natural soft switching in a closed loop to regulate the output. A resonant converter using PFM senses the power supply output and controls it by varying the switching frequency. An advantage of a resonant converter with PFM is that with the natural soft switching during normal operation, there is reduced switching loss compared to non-resonant power converter topologies. Another advantage is that resonant converter with PFM can be designed to operate at higher frequencies and in a smaller package sizes, than PWM converters, generally speaking.
Among a variety of resonant switched mode power converters are high frequency (HF) transformer isolated LLC converters, which have become increasingly popular in recent years. LLC resonant converters utilize the resonance between two inductors and a capacitor to provide natural soft switching. LLC resonant converters save on cost and size by utilizing the magnetizing and leakage inductance of the HF transformer as part of their resonance components. One disadvantage of some resonant converters is that the required wide range of frequency control result in more complicated electromagnetic interference (EMI) filter designs. However, with the gain characteristics of LLC resonant converters, output regulation can be achieved with a narrow band of frequency control.
While soft switching provides advantages during normal operation, this is not the case during startup, when the resonant converter is first started up and there is no energy in the LLC circuit. However, conventional resonant converters simply start symmetric switching at startup, and settle in response to feedback signals. Symmetric switching refers to alternately switching the upper and lower switches with equal on-times. However, rather than the soft switching that occurs with symmetric switching during normal operation, where there is energy in the resonant elements, during startup there can be hard, high current switching that must be addressed to prevent damage to the switching devices and other circuitry. Accordingly, the switching components have to be robust enough to deal with hard switching events during startup, which typically requires components that are larger than is necessary for normal operation. This is especially true for resonant converters that experience numerous startup events, such as in a device that is often turned on and off.
Accordingly, there is a need for a method and apparatus for reducing avoiding hard switching conditions at startup in a resonant converter.
Those skilled in the field of the present disclosure will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of embodiments of the present invention.
The apparatus and method components have been represented where appropriate by conventional symbols in the drawings, showing only those specific details that are pertinent to understanding the embodiments of the present invention so as not to obscure the disclosure with details that will be readily apparent to those of ordinary skill in the art having the benefit of the description herein. Well known elements, structure, or processes that would be necessary to practice the invention, and that would be well known to those of skill in the art, are not necessarily shown and should be assumed to be present unless otherwise indicated.